1. Field of the Invention
The present invention relates to a color image readout apparatus that is applicable to an apparatus such as a copying machine, an image scanner, or a film scanner for reading out a color manuscript or a color image by using a solid image pickup device such as a CCD (a Charge Coupled Device), and to a color image readout lens used therein.
2. Description of the Related Art
A color image read-out lens used for copying machines and various type scanners is preferred to well correct various aberrations without light vignetting in the range from the center of the lens to the peripheral portion thereof, and to have uniformly high resolving power in an overall image height. Particularly, to obtain high resolution, it is important to precisely correct lateral chromatic aberration and longitudinal chromatic aberration so as to remove magnification difference in image formation and contrast difference in resolution for each of colors of B (blue), G (green), and R (red). In correction of the longitudinal chromatic aberration, a normal achromatic lens can bring specified two wavelengths into focus at the same point with respect to primary spectrum, but in the other wavelengths, chromatic aberration remains as secondary spectrum. Accordingly, contrast difference in resolution depending on wavelengths occurs, and thus high resolution is not obtained in the whole range of BGR. By using an anomalous dispersion glass, the secondary spectrum can be corrected, but these kinds of glass usually have high material cost, or are usually disadvantageous in cost in that a processing for the glass is difficult. Thus, in order to completely correct aberrations with high accuracy, a method of increasing the number of lenses is required.
Recently, in general, there is known an example of using a diffractive optical element (DOE) in an optical system as means for correcting chromatic aberration by diffraction effect. FIGS. 32 and 33 illustrate exemplary configurations of kinoform type diffractive optical elements, as examples of diffractive optical elements. The diffractive optical element diffracts rays passing therethrough by forming a plurality of saw-like steps concentrically on a surface of a substrate 101. In the diffractive optical element, a plurality of orbicular zones 102 is formed in a front view as shown in FIG. 33. In the front view, most part of the orbicular zones 102 except for a center circular region 103 is diffractive.
Contrary to general lens materials, such a diffractive optical element has a negative Abbe number and large dispersion. Hence, it is possible to excellently correct chromatic aberration by appropriately combining the diffractive optical element with a normal refractive lens system (a lens system that does not use a diffractive optical element). Known lens systems using a diffractive optical element is disclosed in JP-A-10-311946, JP-A-10-339843, JP-2000-66093 and JP-2007-94278.
However, in the lens system described in JP-A-10-311946 and JP-2007-94278, the diffractive structure is formed on a surface having a large curvature. Hence, as compared with a case where the diffractive structure is formed on a flat surface, an effect of a shape error of the structure such as axial deviation between a vertical direction and an optical axis of the surface and astigmatism of XY directions orthogonal to an optical axis increases. Therefore, it is possible to expect deterioration in resolving power of the lens system. Hence, high accuracy is required to mold the lens system. In addition, in another case, any one of a surface serving as a substrate and a surface opposite thereto is formed as an aspheric surface. Hence, as compared with a case where the diffractive structure is formed on a flat surface or a spherical surface, an effect of a shape error in a manufacturing process such as axial deviation between a vertical direction and an optical axis of the surface, astigmatism of XY directions, and additionally an aspheric surface shape error (a surface undulation) in the optical axis direction greatly increases. Therefore, it is possible to expect further deterioration in resolving power of the lens system. In addition, the lens system described in JP-A-10-339843 includes many lenses, and thus has high price and is insufficient in miniaturization. In addition, in JP-2000-66093, there is an example in which a diffractive structure is formed on a flat substrate, but the substrate has too many orbicular zones of the diffractive structure. Hence, there is a great effect of a processing error, and it is difficult to form a desired shape with high accuracy. In addition, since a space of an aperture diaphragm is too wide, a decrease in size is not achieved.